1. Field of the Invention
Generally, the present disclosure relates to the manufacture of sophisticated semiconductor devices, and, more specifically, to various methods of forming transistor devices with retrograde wells in CMOS (Complementary Metal Oxide Semiconductor) applications, and the resulting device structures.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPU's, storage devices, ASIC's (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout, wherein so-called metal oxide field effect transistors (MOSFETs or FETs) represent one important type of circuit element that substantially determines performance of the integrated circuits. A FET is a device that typically includes a source region, a drain region, a channel region that is positioned between the source region and the drain region, and a gate electrode positioned above the channel region. Current flow through the FET is controlled by controlling the voltage applied to the gate electrode. If a voltage that is less than the threshold voltage of the device is applied to the gate electrode, then there is no current flow through the device (ignoring undesirable leakage currents, which are relatively small). However, when a voltage that is equal to or greater than the threshold voltage of the device is applied to the gate electrode, the channel region becomes conductive, and electrical current is permitted to flow between the source region and the drain region through the conductive channel region. The above description is applicable for both the N-type FET as well as the P-type FET, except that the polarity of voltage in operation and the doping type of the source, the channel and the drain regions are correspondingly reversed. In so-called CMOS (Complementary Metal Oxide Semiconductor) technology, both N-type and P-type MOSFETs (which are referred to as being “complementary” to each other) are used in integrated circuit products. CMOS technology is the dominant technology as it relates to the manufacture of almost all current-day large scale logic and memory circuits.
To improve the operating speed of FETs, and to increase the density of FETs on an integrated circuit device, device designers have greatly reduced the physical size of FETs over the years. More specifically, the channel length of FETs has been significantly decreased, which has resulted in improving the switching speed of FETs. However, decreasing the channel length of a FET also decreases the distance between the source region and the drain region. In some cases, this decrease in the separation between the source and the drain makes it difficult to efficiently inhibit the electrical potential of the channel from being adversely affected by the electrical potential of the drain, which is commonly referred to as a “punch-through” of the electrical potential from the drain to the source and leads to larger leakage currents. This is sometimes referred to as a so-called short channel effect, wherein the characteristic of the FET as an active switch is degraded.
In contrast to a planar FET, which has a planar structure, there are so-called three-dimensional (3D) devices, such as an illustrative FinFET device, which is a three-dimensional structure. More specifically, in a FinFET, a generally vertically positioned, fin-shaped active area is formed and a gate electrode encloses both of the sides and the upper surface of the fin-shaped active area to form a “tri-gate” structure so as to use a channel having a 3D “fin” structure instead of a planar structure. In some cases, an insulating cap layer, e.g., silicon nitride, is positioned at the top of the fin and the FinFET device only has a dual-gate structure. Unlike a planar FET, in a FinFET device, a channel is formed perpendicular to a surface of the semiconducting substrate so as to reduce the depletion width in the “fin” channel (as a result of the better electrostatic characteristics of the tri-gate or dual-gate structure around the fin channel) and thereby reduce so-called short channel effects. Also, in a FinFET, the junction capacitance at the drain region of the device is greatly reduced, which tends to reduce at least some short channel effects.
In one embodiment, FinFET devices have been formed on so-called silicon-on-insulator (SOI) substrates. An SOI substrate includes a bulk silicon layer, an active layer and a buried insulation layer made of silicon dioxide (a so-called “BOX” layer) positioned between the bulk silicon layer and the active layer. Semiconductor devices are formed in and above the active layer of an SOI substrate. The fins are formed in the active layer and the buried insulation layer provides good isolation between adjacent fins. The processes used to form FinFET devices on SOI substrates have relatively good compatibility with various processes that are performed when forming planar transistor devices in CMOS applications. For example, in both applications, the gate stack and the gate insulation layer can be made of the same materials (as in planar CMOS on SOI), e.g., poly-SiON or high-k/metal-gate (HKMG), and both applications may involve performing various epitaxial silicon growth processes (e.g., SiGe for PMOS and raised SD for NMOS) as well as the formation of epi-silicon material on the fins so as to define the source/drain regions from the FinFET devices that provide good resistance and desirable stress characteristics. When an appropriate voltage is applied to the gate electrode of a FinFET device, the surfaces (and the inner portion near the surface) of the fins, i.e., the substantially vertically oriented sidewalls and the top upper surface of the fin with inversion carriers, contributes to current conduction. In a FinFET device, the “channel-width” is approximately two times (2×) the vertical fin-height plus the width of the top surface of the fin, i.e., the fin width. Multiple fins can be formed in the same foot-print as that of a planar transistor device. Accordingly, for a given plot space (or foot-print), FinFETs tend to be able to generate significantly stronger drive current than planar transistor devices. Additionally, the leakage current of FinFET devices after the device is turned “OFF” is significantly reduced as compared to the leakage current of planar transistor MOSFETs due to the superior gate electrostatic control of the “fin” channel on FinFET devices. In short, the 3D structure of a FinFET device is a superior MOSFET structure as compared to that of a planar MOSFET, especially in the 20 nm CMOS technology node and beyond.
The formation of transistor devices in CMOS technology has also evolved and continues to evolve to produce devices with improved operational characteristics. One relatively recent advance involves the use of low channel doping (i.e., super-steep channel doping profiles) for deeply depleted channel regions during device operation, where there are multiple epi layers (i.e., Boron-doped-Silicon (Si:B), Carbon-doped Silicon (Si:C) and non-doped Silicon) formed above N/P wells. In such a device, the suppression of boron (B), phosphorous (P) and arsenic (As) diffusion is mainly due to the presence of the carbon-doped silicon layer (Si:C) layer. Alternatively, instead of using epitaxial growth processes, the B-doped and C-doped silicon layers can be formed by implanting boron and carbon into the silicon substrate in both the N and P active regions of the substrate. The low doping of the channel region may suppress or reduce the so-called “short-channel effect” typically found on traditional planar transistor devices manufactured on bulk silicon, reduce variations in the threshold voltages of such devices (due to less random dopant fluctuations), reduce source/drain leakage currents (by punch-through suppression by those doped layers below the channel) and lower junction capacitances. Therefore, MOSFET devices formed on a bulk substrate with a low doped channel can enjoy the advantages of devices with fully depleted channel regions during operations as if they are fabricated on an SOI substrate.
It is generally known that fully depleted devices with a substantially un-doped or low-doped channel region are effective in reducing threshold voltage variability due to the elimination of random dopant fluctuations in such devices, and that such devices exhibit improved device performance with relatively low dynamic power requirements, low leakage currents and relatively high transistor density. The fully depleted devices can take the form of planar transistor devices with ultra-thin bodies formed on SOI substrates or three-dimensional devices, such as FINFET devices. However, the planar devices consume a substantial amount of plot space (or foot-print) in the channel width direction and, with respect to FINFET technology, there are significant challenges in forming deep fin/isolation trenches and filling such trenches without creating undesirable voids.
The present disclosure is directed to various methods of forming transistor devices with retrograde wells in CMOS applications, and the resulting device structures, that may solve or reduce one or more of the problems identified above.